1. Field of the Invention
The present invention relates to an electrode for a fuel cell which is capable of achieving a high output at a high current density, a method of manufacturing such an electrode, and a fuel cell having such an electrode.
2. Description of the Related Art
FIG. 9 of the accompanying drawings shows in vertical cross section a unit cell of a general phosphoric acid fuel cell which is one of presently available fuel cells. The unit cell, denoted at 1, has an electrolyte electrode assembly 5 which comprises an anode electrode 2, a cathode electrode 3, and an electrolyte 4 interposed between and joined to the electrodes 2, 3.
The electrolyte 4 comprises a polymer membrane made of a basic polymer such as polybenzimidazole and impregnated with a liquid electrolyte. See U.S. Pat. No. 5,525,436 for details. The liquid electrolyte may, for example, be phosphoric acid, sulfuric acid, methanesulfonic acid, or the like which conducts hydrogen ions.
As shown in FIG. 10 of the accompanying drawings, the anode electrode 2 and the cathode electrode 3 comprise respective gas diffusion layers 6a, 6b and respective electrode catalyst layers 7a, 7b coated uniformly on respective surfaces of the gas diffusion layers 6a, 6b. Generally, the gas diffusion layers 6a, 6b are made of carbon paper, carbon cloth, or the like, and the electrode catalyst layers 7a, 7b are made of a carbon black with a catalyst of Pt carried on its particle surface, or the like.
As shown in FIG. 9, the electrolyte electrode assembly is interposed between two separators 8a, 8b. The unit cell 1 also has collector electrodes 9a, 9b held against respective outer surfaces of the separators 8a, 8b, and end plates 10a, 10b held against respective outer surfaces of the collector electrodes 9a, 9b. The end plates 10a, 10b are connected to each other by bolts (not shown), sandwiching the electrolyte electrode assembly 5, the separators 8a, 8b, and the collector electrodes 9a, 9b between the end plates 10a, 10b. The separators 8a, 8b have respective gas passages 11a, 11b defined therein for supplying a hydrogen-containing gas and an oxygen-containing gas to the anode electrode 2 and the cathode electrode 3.
The anode electrode 2, the cathode electrode 3, and the electrolyte 4 are accommodated respectively in frame-shaped seals 12, 13, 14.
The general phosphoric acid fuel cell comprises a stack (not shown) of unit cells 1 that are electrically connected in series, a mechanism for supplying a hydrogen-containing gas to and discharging a hydrogen-containing gas from the stack, and a mechanism for supplying an oxygen-containing gas to and discharging an oxygen-containing gas from the stack.
For operating the phosphoric acid fuel cell, a fuel gas such as a hydrogen-containing gas or the like is supplied via the gas passages 11a in the separator 8a to the anode electrode 2 of each unit cell 1, whereas an oxygen-containing gas such as air or the like is supplied via the gas passages 11b in the separator 8b to the cathode electrode 3. The fuel gas and the oxygen-containing gas pass through the respective gas diffusion layers 6a, 6b of the electrodes 2, 3 and then reach the respective electrode catalyst layers 7a, 7b. In the electrode catalyst layer 7a of the anode electrode 2, the hydrogen in the fuel gas causes a reaction represented by the following formula (A), generating hydrogen ions and electrons:2H2→4H++4e  (A)
The generated hydrogen ions move through the electrolyte 4 to the cathode electrode 3. During this time, the electrons flow to an external circuit that is electrically connected to the anode electrode 2 and the cathode electrode 3, are used as an electric energy in the form of a direct current to energize the external circuit, and then flow to the cathode electrode 3.
The hydrogen ions that have moved to the cathode electrode 3 and the electrons that have moved to the cathode electrode 3 via the external circuit react with the oxygen contained in the oxygen-containing gas supplied to the cathode electrode 3, as indicated by the following formula (B):O2+4H++4e→2H2O  (B)
The fuel gas which remains unreacted is discharged out of the unit cell 1 (fuel cell) through the gas passages 11a in the separator 8a. Similarly, the oxygen-containing gas which remains unreacted and the generated H2O are discharged out of the unit cell 1 through the gas passages 11b in the separator 8b. 
If the voltage across the unit cell 1 is represented by V, the density of the current generated by the unit cell 1 by I, and the effective area of the electrodes 2, 3 by S, then the output P of the unit cell 1 is determined according to the following equation (C):P=I×V×S  (C)
In the fuel cell, as the current density I increases, the cell voltage V drops, and finally the output P of the unit cell 1 drops, as can be seen from the equation (C). Specifically, when the fuel cell generates electricity at a large current density, the voltage across the fuel cell drops below a desired level, failing to sufficiently energize the load connected to the fuel cell. For this reason, there has been a demand for a unit cell whose voltage drop is lower even when it generates electricity at a large current density, i.e., whose output is large at a large current density.
While the cell voltage V across the unit cell 1 varies depending on the current density I, the cell voltage V is generally about 1 V. If a higher cell output is needed to provide a power supply for a motor on an automobile, for example, then it is necessary to connect a number of unit cells 1 in series. However, the resulting fuel cell increases in weight and size, requiring a large installation space on the automobile. The automobile with the fuel cell carried thereon also becomes heavy. To avoid these shortcomings, there has been a demand for a unit cell with a higher output.
For increasing the cell voltage V at the time the unit cell is discharged at a high current density, it is necessary to reduce the internal resistance of the unit cell 1.
As can be understood from the formula (B), H2O is generated during operation of the fuel cell. The amount M (mol/min.) of H2O generated per unit time is determined from the current density I (A/cm2) and the effective electrode area S (cm2) according to the following equation (D):M=I×S×60/(96500×2)  (D)
Thus, the amount M of H2O generated per unit time increases as the current density I increases. In continued operation of the fuel cell under this condition, when the generated H2O is discharged, the liquid electrolyte of phosphoric acid or the like seeps from the composite electrolyte 4, tending to clog the interstices of the gas diffusion layer 6b (carbon paper or the like). When the interstices of the gas diffusion layer 6b clogged, it is difficult for the supplied oxygen-containing gas to be diffused to the electrode catalyst layer 7b, making it difficult for the reaction represented by the formula (B) to take place. Stated otherwise, since the reaction efficiency of the reaction represented by the formula (B) is lowered, the cell voltage of the unit cell drops.
Japanese laid-open patent publication No. 60-170168 proposes a water-repellent layer interposed between a gas diffusion layer and an electrode catalytic layer for repelling a liquid electrolyte to prevent the liquid electrolyte from seeping from an electrolyte.
The water-repellent layer is produced by coating a gas diffusion layer of carbon paper or the like with a solution which has been prepared by dissolving a porous carbon material and PTFE (polytetrafluoroethylene) in a solvent such as water, isopropyl alcohol, or the like, and heating the coated gas diffusion layer preferably at a temperature ranging from 350 to 400° C. to harden the coated solution.
However, the water-repellent layer thus produced tends to crack easily to the extent that the produced cracks usually reach about 5%, or about 15% at maximum, of the total area of the water-repellent layer. The water-repellent layer itself may suffer surface irregularities and have maximum and minimum thicknesses that differ from each other by at least 60 μm. When the coated gas diffusion layer is heated in the above temperature range, the water-repellent layer becomes too water-repellent to allow an electrode catalyst layer paste to be easily coated on the gas diffusion layer. An electrode catalyst layer somehow coated on the gas diffusion layer also tends to crack and suffer surface irregularities. The cracks in the electrode catalyst layer usually reach about 10%, or about 30% at maximum, of the total area of the electrode catalyst layer. The electrode catalyst layer with surface irregularities has maximum and minimum thicknesses that differ from each other by about 50 μm.
The crack in the electrode catalyst layer obstructs electric conduction in the electrode catalyst layer. The electrode catalyst layer with the surface irregularities suffer an irregular charge distribution. At any rate, the electric conductivity of the electrodes is lowered, resulting in an increase in the internal resistance of the fuel cell.
As described above, when the fuel cell is operated to generate electricity at a high current density, the liquid electrolyte seeps from the composite electrolyte. The water-repellent layer added to prevent the liquid electrolyte from seeping from the composite electrolyte invites an increase in the internal resistance of the fuel cell. Consequently, it is highly difficult to construct a fuel cell which is capable of achieving a high output at a high current density.